This invention pertains to production of hydrogen peroxide by catalytic direct synthesis from hydrogen and oxygen-containing feedstreams. It pertains particularly to a process for directly producing hydrogen peroxide (H2O2) product utilizing an active supported noble metal phase-controlled catalyst in a liquid medium containing an organic solvent and water for providing high activity and product selectivity to the process, and can utilize feedstreams containing low safe hydrogen concentrations below their lower flammability limit.
Demand for hydrogen peroxide product has been growing globally at about 6% annually, and in North America at about 10% annually. Such demand growth is due primarily to the enviromnental advantages of hydrogen peroxide usage, which upon decomposition releases only oxygen and water. Hydrogen peroxide is an effective replacement for chlorine in pulp and paper bleaching, water treatment and other environmental processes, and meets the growing product demand and need for a simple environmentally friendly and cost effective process that can be located on-site for the pulp, paper and other manufacturing facilities. The hydrogen peroxide presently being produced commercially uses a known anthraquinone process which has low yields and some safety problems. Also, transportation of hydrogen peroxide from a production site to an end-user facility is an important safety issue due to the risk of explosion of hydrogen peroxide by its violent decomposition.
Many attempts have been made to produce hydrogen peroxide directly from hydrogen and oxygen-containing feedstreams, because such a process not only has potential for significantly reducing production cost, but also provides an alternative production process which avoids the present use of toxic feedstock and working solutions. For such direct catalytic production of hydrogen peroxide, the feedstreams are hydrogen and air which are clean and environmentally harmless. Such direct catalytic process generates no waste and is cost efficient due to its inherent simplicity, and the hydrogen peroxide product can be used directly as a bleaching agent in pulp and paper processes. However, such proposed direct production technology has not yet been commercialized, as the major problems for the known such processes are (1) hazardous operating conditions (with the feed hydrogen partial pressure within the flammable or explosive range), (2) low reaction rates, and (3) low catalytic product selectivity.
Although the direct catalytic synthesis of hydrogen peroxide product has attracted much attention and many patents have been issued, none of the patented processes have been commercially feasible due to low catalyst activity and low selectivity for the hydrogen peroxide product. Until the early 1990""s most of these patents utilized as feed gas at least 10% hydrogen in air or oxygen, which is within the flammabiltiy limits for the H2/O2 mixture. Due to increasing safety concerns, the recent approach has been to utilize feedstreams having hydrogen concentration below about 5 vol. %. However, at such low hydrogen concentration, the catalysts used must be much more active to achieve an acceptable production rate for hydrogen peroxide. Highly dispersed palladium on various support materials has been used to enhance the catalytic activity. However, the dispersion methods used have not adequately controlled the crystal phase of the palladium, and the desired improvement in selectivity towards hydrogen peroxide product has not been achieved. A main problem in preparing a highly selective catalyst for hydrogen peroxide production is how to consistently control the formation of desired metal phase such as phase 110 or 220, etc. in the catalyst.
Most of the known prior processes for direct hydrogen peroxide catalytic synthesis are based on use of an aqueous liquid medium for conducting the synthesis reaction, as hydrogen peroxide is generally produced commercially as an aqueous product. Use of organic compounds in combination with hydrogen peroxide can raise safety concerns related to the unintended formation of organic peroxides which can be fire or explosion hazards, especially if accidentally concentrated for example by precipitation. However, there are some prior art patents disclosing direct synthesis of hydrogen peroxide in liquid mediums that include an organic solvent. One class of such prior art processes involves the use of a liquid medium consisting of a two-phase mixture of water and an organic solvent which is immiscible with water. In general, the operating principle of such prior art processes is that the peroxide synthesis catalyst is contained in the organic phase, such that hydrogen peroxide synthesis occurs in this phase. But the resulting hydrogen peroxide product is poorly soluble in that phase, so the peroxide is extracted into the aqueous phase, segregating the product from the catalyst and preventing undesired product degradation.
U.S Pat. No. 4,128,627 discloses hydrogen peroxide being synthesized in a two-phase mixture using a homogeneous palladium-based catalyst which is insoluble in water, with preferred organic solvents being 1,2-dichlorobenzene, chlorobenzene and xylene. A critical function of the organic solvent component is to dissolve the homogeneous catalyst, which is insoluble in the aqueous phase. The best results reported are a hydrogen peroxide product concentration of only 0.45 wt % and a product yield of only 11.59 g H2O2/g Pd/hr, but requiring an undesirably high hydrogen feed concentration of 97.2 vol. %. In U.S. Pat. No. 4,336,240, it is disclosed that when the organic solvent is a fluorocarbon or halofluorocarbon such as 1,1,2-trichloro-trifluoroethane, a somewhat higher hydrogen peroxide product concentration of 3.2 wt % is achieved, but at a reduced yield of only 0.99 g H2O2/g Pd/hr, and again with very high hydrogen concentration in the feed gas.
U.S. Pat. Nos. 4,347,231 and 4,347,232 utilize the same two-phase liquid medium concept using homogeneous iridium-based and palladium-based catalysts, respectively, and preferred organic solvents are toluene, xylene, and chlorinated solvents such as dichloromethane. Again, the key operating principle is that the organic solvent is present to dissolve the water-insoluble homogeneous catalyst, and the water phase is present to extract the peroxide product away from the organic phase. The best results were 1.7% H2O2 product concentration and 89 g H2O2/g Pd/hr yield, but with undesired high hydrogen feed concentrations of 50 vol. % which are well above the explosion limit.
For U.S. Pat. No. 5,399,334 a two-phase liquid reaction medium is used, wherein the organic solvent is a halogenated organic, especially hydrocarbons substituted by at least three fluorine atoms. The best results reported were only 0.8 wt. % H2O2 product concentration at a yield of 266 g H2O2 g Pd/hr, or 3.5 wt % H2O2 product concentration at a yield 194 g H2O2/g Pd/hr.
Another group of prior art processes in which organic solvents are used as at least part of the liquid medium for direct catalytic hydrogen peroxide synthesis is those patents where only a single liquid phase is present. For example, U.S. Pat. No. 3,361,533 utilizes a liquid mixture of water with a soluble organic solvent such as alcohol or ketone, with acetone being mentioned as the best organic solvent, and the catalyst is a heterogeneous supported noble metal, especially palladium (Pd). A high hydrogen feed concentration of 16.7 vol. % is used, which is well above the flammability limit and close to the explosion limit, but the hydrogen peroxide yield was only 4.86 g H2O2 g Pd/hr.
U.S. Pat. No. 4,007,256 utilizes a one-phase liquid reaction medium consisting of water mixed with an organic nitrogen-containing compound such as acetonitrile, and a supported palladium catalyst. A high hydrogen feed concentration of 50 vol. % was used, again well above the explosive limit, and the best hydrogen peroxide product concentration was 6.4 wt %, with a product yield of 160 g H2O2/g Pd/hr.
U.S. Pat. No. 4.335,092 uses a liquid reaction medium of primarily methanol with a small amount of formaldehyde, with the catalyst being supported palladium. Although the gas-phase hydrogen feed concentration was a safe level of 4.2 vol. %, the product hydrogen peroxide concentration was only 1.7 wt %, with a yield of only 12.1 g H2O2/g Pd/hr.
U.S. Pat. No. 4,336,239 utilizes a reaction liquid comprising a mixture of water and an organic solvent containing oxygen or nitrogen. Acetone is the preferred solvent, and the catalyst is a supported noble metal such as palladium. An undesirably high hydrogen gas-phase feed concentration of 22.6 vol. % was used, and the best hydrogen peroxide product concentration reported was 3.4 wt %, at a yield of 94 g H2O2/g Pd/hr.
It is apparent that while the prior art discloses use of liquid reaction medium for catalytic hydrogen peroxide synthesis including at least in part an organic solvent, the performance results of these prior processes for hydrogen peroxide product concentration and product yield are not notably better than most results reported for the direct catalytic synthesis of hydrogen peroxide in a purely aqueous liquid medium. Moreover, the most promising results were generally obtained using dangerously high hydrogen gas-phase feed concentrations.
The present invention provides a significantly improved process for catalytic direct synthesis of hydrogen peroxide (H2O2) product from hydrogen and oxygen-containing feeds, utilizing an active supported noble-metal phase-controlled catalyst in combination with a liquid medium containing at least some organic solvent, which combination of catalyst and liquid solvent provides unexpectedly large improvements in hydrogen peroxide concentration and yield as compared to utilizing a purely aqueous liquid medium. The particulate noble metal catalyst useful in this invention is insoluble in the liquid medium. The preferred supported noble metal phase-controlled catalyst of this invention includes a particulate support material having total surface area of 50-500 m2/gm; and 0.01-10 wt. % noble metal controllably deposited on the particulate support material, the noble metal having a wide distribution of minute crystals each having size of 0.5-100 nanometers (nm), and atoms of the noble metal being exposed in an orderly linear alignment pattern on the support material, so that at least most of the noble metal crystals have a phase exposition of 110 and/or 220, with the noble metal being palladium, which can be used in combination with platinum, gold, iridium, osmium, rhodium, or ruthenium, and combinations thereof. This preferred catalyst is disclosed in our U.S. Pat. No. 6,168,775, which is being incorporated herein by reference to the extent necessary to adequately disclose the present invention. For this preferred catalyst, the noble metal constituent is present as nano-size particles having a controlled phase exposition, thereby assuring that only the most active and selective noble metal catalytic sites are available for reaction with the liquid solvent medium.
A critical feature of this invention is the unexpected discovery of a significant performance enhancement achieved by conducting the catalytic direct synthesis reaction in a liquid medium including, at least in part, a selected organic solvent. This solvent solution discovery is contrary to the teachings of the prior art, from which no significant improvement in product concentration or yield would be suggested by using a organic solvent reaction medium for catalytic direct hydrogen peroxide synthesis of hydrogen peroxide product. Although a variety of known organic solvents may be used in this invention, the appropriate solvent selection is influenced by various factors, including catalyst performance enhancement, ease of separating the liquid solvent from the peroxide-containing liquid product for recycle, ultimate use for the hydrogen peroxide product, and the possibility of side reactions occurring between the solvent and the hydrogen peroxide which might form undesirable non-selective products or pose a safety hazard. The organic solvent may be used as a pure solvent, or as a mixture with water, with the selection related to similar factors as defined by a unique Solvent Selection Parameter (SSP). The Solvent Selection Parameter is defined based on the solubility of hydrogen in the solvent, and is specifically defined as follows:
Solvent Selection Parameter=xcexa3(wixc3x97Si)
where:
wi is the weight fraction of solvent component i in the liquid reaction mixture,
Si is the solubility of hydrogen in pure component i, expressed as mole fraction at standard conditions of 25xc2x0 C. and 1 atm, and
the symbol xcexa3 indicates a sum over all of the components that comprise the liquid reaction mixture.
This Solvent Selection Parameter (SSP) is simple to calculate based on hydrogen solubility data that are available in the open literature. Although this Solvent Selection Parameter takes no account of non-linear changes in hydrogen solubility that may occur upon mixing different liquids, it has been found to be very useful in selection of appropriate organic solvents for the liquid medium for the practice of this invention
This Solvent Selection Parameter of this invention has been found to correlate strongly to a key measure of process performance, namely the catalyst hydrogen peroxide yield, which is defined as the weight of hydrogen peroxide produced per weight of active noble metal per hour. For a series of liquid reaction mixtures comprising water, pure organic solvent, or mixtures of water and solvent, the Solvent Selection Parameter was calculated, and the catalyst hydrogen peroxide yields were measured in laboratory catalyst performance tests. These data results are shown numerically in Table 1, and are also shown graphically in FIG. 1.
As evident in FIG. 1, there is a strong linear correlation between the Solvent Selection Parameter (SSP) and the catalyst hydrogen peroxide yield, with improved yield being achieved as the Solvent Selection Parameter is increased. The comparative benchmark is the use of water alone as the liquid reaction medium, which has a Solvent Selection Parameter of 0-14xc3x9710xe2x88x924, and gives a catalyst hydrogen peroxide yield of 207 g H2O2/g Pd/hr in performance test. By using different solvents or solvent/water mixtures that have higher Solvent Selection Parameters, higher yields up to about 900 g H2O2/g Pd/hr can be achieved. These results demonstrate that increased hydrogen solubility in the solvent medium is a controlling factor that improves the hydrogen peroxide concentration and yield. For the purposes of this invention, the liquid reaction medium will have a Solvent Selection Parameter that is greater than 0.14xc3x9710xe2x88x924, and not exceeding about 5.0xc3x9710xe2x88x924. Preferred liquid solvents will have a Solvent Selection Parameter between 0.2xc3x9710xe2x88x924 and 4.0xc3x9710xe2x88x924.
While FIG. 1 shows a generally linear increase in catalyst hydrogen peroxide yield with increases in the Solvent Selection Parameter (SSP), such an increase is not sustained indefinitely. An upper limitation has been discovered for appropriate values of the Solvent Selection Parameter for the practice of this invention. This limitation derives from the fact that the preferred solvents should be soluble in water, and that the liquid reaction mixture should comprise a single liquid phase. Organic solvents with the highest hydrogen solubility are generally those which are highly hydrophobic, including widely used solvents like paraffinic hydrocarbons such as hexane and the like, and aromatic hydrocarbons such as benzene, toluene, and the like. While liquid reaction mixtures comprising all or part of solvents of this type have relatively high Solvent Selection Parameter values, they are not preferred for the practice of this invention because they have poor miscibility with water. Hydrogen peroxide is not sufficiently soluble in these solvents, thereby hindering the critical step of product desorption from the catalyst surface into the surrounding liquid medium. This desorption problem causes the hydrogen peroxide product to remain at or near the catalyst surface, where it tends to undergo further chemical reaction to form undesired water by-product, resulting in poor catalyst hydrogen peroxide yields. Therefore, for the practice of this invention, the liquid reaction medium should have a Solvent Selection Parameter (SSP) values less than 5.0xc3x9710xe2x88x924, and preferably less than 4.0xc3x9710xe2x88x924.
Useful organic solvents for this invention include oxygen-containing compounds such as alcohols, ketones, aldehydes, furans (e.g. THF), ethers, and esters, nitrogen-containing compounds such as nitrites, amines, and amides (e.g. DMF), phosphorus containing compounds such as organic phosphine oxides (e.g. Cyanex products produced by Cytec), hydrocarbons such as aliphatic hydrocarbons and aromatic hydrocarbons, and the like, or mixtures thereof. Preferred solvents are those which are miscible with water and have good solubility for hydrogen peroxide, because it has been found in the practice of this invention that a one-phase liquid reaction medium provides superior yield results. Furthermore, it is preferred that the solvent have a boiling point temperature lower than that of water or hydrogen peroxide, which allows the solvent to be recovered from the peroxide-containing product as an overhead stream by a distillation step. Such lower boiling temperature relationship avoids the need to distill hydrogen peroxide overhead from a heavier solvent, which is a hazardous operation. Examples of preferred solvents are light alcohols such as ethanol, methanol, n-propanol and isopropanol, light ketones such as acetone, and nitrogen-containing solvents such as acetonitrile and 1-propylamine.
In the process of this invention, the yield of hydrogen peroxide based on the catalyst may be improved by the addition of a suitable promoter to the reaction medium. Examples of effective promoters are halide salts such as sodium bromide, sodium chloride, sodium iodide, and the like. By adding a halide salt in an amount in the range of 1 ppm to 500 ppm by weight of the liquid reaction medium, and preferably 3 ppm to 200 ppm, the catalyst hydrogen peroxide yield can be substantially improved.
Referring to FIG. 1, it is evident that the addition of a promoter is only effective when the desired concentration of promoter is fully soluble in the liquid mixture. For the data points along the upper curve xe2x80x9cAxe2x80x9d of FIG. 1, 5 ppm by weight of sodium bromide (NaBr) was added to the liquid mixture. The solubility of NaBr in these liquid mixtures was greater than 5 ppm, so that the amount of added NaBr dissolved completely. In these cases, the catalyst hydrogen peroxide yield rises rapidly as the Solvent Selection Parameter (SSP) is increased, so that greater than a four-fold increase in yield is achieved relative to the comparative case of using only water as the liquid reaction solvent by increasing the Solvent Selection Parameter from 0.14 to 1.6.
In cases where promoters such as halide salts are either not used or are insoluble in the liquid solvent mixture, lesser results are achieved as shown by to the lower curve xe2x80x9cBxe2x80x9d of FIG. 1. In these cases, increases in SSP also result in improved catalyst hydrogen peroxide yield, but the rate of increase is lower than when the NaBr promotor is used. However, catalyst hydrogen peroxide yields achieved for higher values of SSP, even in the absence of a promoter, are substantially greater than those achieved at low values of SSP with a promoter. Relative to the comparative case of using water as the reaction medium with NaBr soluble promoter, catalyst hydrogen peroxide yields in the absence of promoter are increased almost four-fold by increasing the SSP value to 2.7xc3x9710xe2x88x924.
Therefore, in the practice of this invention utilizing the desired Solvent Selection Parameter (SSP) values, substantial improvements in catalyst hydrogen peroxide yields are advantageously achieved relative to the known prior art processes, either with or without use of promoters such as halide salts in the reaction medium. By using such preferred promoters in combination with liquid mixtures in which they are soluble, higher catalyst hydrogen peroxide yields are achieved. Such use of soluble promotors can advantageously result in smaller reactor size and reduced catalyst requirement, which lowers capital and operating costs for the process. However, depending on the ultimate use for the hydrogen peroxide product, the presence of such promoters in the product may not be acceptable, and would require separation of the promoter from the reaction product, which would add some cost and complexity to the process.
While the liquid reaction medium may comprise an essentially pure organic solvent without water, it is preferable to conduct the hydrogen peroxide synthesis in a reaction medium which contains a portion of water. In commercial practice, the solvent fed to the catalytic peroxide synthesis reactor will be recovered and recycled back to the reactor from a point downstream in the process, and it is preferable to avoid any need to purify this solvent to a high degree, but instead to allow a fraction of water to be recycled along with the solvent, which reduces costs for distillation or other separations. Also, hydrogen peroxide is typically produced and marketed as an aqueous solution. If the purpose of the hydrogen peroxide produced by this process is commercial sale, then upon removal and recycle of the organic solvent, the presence of water in the reaction mixture will lead to the formation of an aqueous hydrogen peroxide solution which is suitable for further processing and commercial use.